(a) Field of the invention
This invention relates to a camera shutter using an electrostrictive strain element as a driving source and more particularly to a lens shutter using, for example, a bimorph type electrostrictive strain element as a driving source.
(b) Description of the prior art
FIGS. 1 and 2 show an example of a conventional lens shutter for cameras using an electrostrictive strain element as a driving source. FIG. 1 shows the shutter as completely closed and FIG. 2 shows the shutter as fully opened. In the drawings, the reference numeral 1 represents a shutter base plate having an exposure aperture 1a and 2 represents a shutter blade for opening and closing the exposure aperture 1a and pivoted on the back surface of the base plate 1 by a shaft 3 and 4 represents a shutter blade opening and closing lever pivoted on the surface of the base plate 1 by a shaft 5 and having a recess portion 4a for receiving the free end of a later described electrostrictive strain element formed in one end portion and an arcuate slot 4b fitted with the shaft 3 provided in the other end portion. The shutter blade opening and closing lever 4 is rotatable only within a range allowed by this slot 4. Pins 4c, 4c are inserted respectively through arcuate slots 1b, 1b of the base plate 1 and are fitted respectively in slots 2a, 2a of shutter blades 2 located on the back surface of the base plate 1 so that, by the pin-slot connection of these pins 4c, 4c. with the slots 2a, 2a the shutter blades 2 may rotate with the rotation of the shutter blade opening and closing lever 4. The reference numeral 6 represents a known bimorph type thin plate-like electrostrictive strain element secured in one end portion to a bracket 7 fitted to the base plate 1 and inserted at the other free end into the recess portion 4a of the shutter blade opening and closing lever 4 with a predetermined clearance l. This clearance l is set by taking the dimensional tolerance and temperature contraction into consideration so that the electrostrictive strain element 6 may not squeak. The electrostrictive strain element 6 is to open and close the shutter blades 2 by engaging at the driving end 6a with either of the first engaging pin 4a-1 and second engaging pin 4a-2. By the way, the above-mentioned clearance l is drawn as exaggerated for the convenience of the illustration.
Therefore, if, for example, a forward voltage is applied from the state shown in FIG. 1 to the electrostrictive strain element 6 and the electrostrictive strain element 6 bends and displaces downward at the driving end 6a, the element 6 will engage at the driving end 6a with the first engaging pin 4a-1 to rotate the shutter blade opening and closing lever 4 clockwise and will open the shutter blades 2 as shown in FIG. 2. If a reverse voltage is applied from the state shown in FIG. 2 to the electrostrictive strain element 6 (or if the electrostrictive strain element 6 is short-circuited between both ends or is earthed to discharge the accumulated charge), the element 6 will engage at the driving end 6a with the second engaging pin 4a-2 to rotate the shutter blade opening and closing lever 4 counterclockwise and will close the shutter blades 2 to return to the state shown in FIG. 1.
In the initial position before the exposure starts as shown in FIG. 1, the driving voltage to the electrostrictive strain element 6 is not applied and the element 6 tends to remain balanced or linear. However, in this initial state, the electrostrictive strain element 6 engages at the driving end 6a with the second engaging pin 4a-2 by its own resiliency and presses the shutter blade opening and closing lever 4 in the blade closing direction. The electrostrictive strain element 6 is shown to be linear in FIG. 1 but should be understood to be somewhat bent in fact.
By the way, in FIGS. 1 and 2, the reference numeral 8 represents a photosensor for detecting a small orifice 2b provided in one shutter blade 2. By the cooperation of the orifice 2b and photosensor 8, the beginning of the opening of the shutter blades 2 is detected and this detecting signal is delivered to a known exposure time measuring means not illustrated to start counting the exposure time.
Now, in the above-mentioned conventional shutter, the total strain amount (total displacement amount) of the driving end 6a of the electrostrictive strain element 6 corresponding to the displacement of the shutter blades 2, 2 from the position shown in FIG. 1 to the fully opened position is shown by L.sub.0 in FIG. 2. However, from the position in which the driving end 6a engages with the second engaging pin 4a-2 of the recess portion 4a until it engages with the first engaging pin 4a-1, the displacement of the electrostrictive strain element 6 does not contribute to the operation of opening the shutter blades 2, 2, therefore the strain amount (displacement amount) of the element 6 substantially required to drive the shutter blades 2, 2 is L.sub.0 -l but the electrostrictive strain element 6 must be strained by L.sub.0 and some percentage of the displacement amount in one direction of the element 6 has been wasted. Therefore, in a range of a large strain amount, that is, in a range low in the reliability of the responding characteristic, the electrostrictive strain element has had to be used to open and close the shutter blades 2, 2. Thus, an electrostrictive strain element large in the strain amount and therefore requiring a costly large driving voltage has had to be used. There have been such disadvantages.